1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an improved fuel rod end plug configuration which facilitates both pulling fuel rods into the top of a fuel assembly from the bottom thereof during initial loading at the manufacturing site and pushing fuel rods into the top of the fuel assembly from the top thereof during subsequent servicing at the reactor facility.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
In assembling a fuel assembly, it has been conventional practice, first, to attach the transverse grids to the longitudinally extending guide thimbles at predetermined axially spaced locations therealong. Next, the fuel rods are loaded through the grids and the bottom nozzle is then attached to the lower ends of the guide thimbles. Last, the top nozzle is attached to the upper ends of the guide thimbles.
Typically, the fuel rods are loaded by pulling them through the grids from the bottom of the fuel assembly. The fuel assembly under construction is located between a fuel rod loader and a fuel rod magazine. At the bottom of the fuel assembly, a gripper is extended outwardly from the fuel rod loader, through the fuel assembly, and to the fuel rod magazine located at the top of the assembly. The gripper is brought into engagement with the lower end plug of a fuel rod stored in the magazine and is then retracted back through the fuel assembly, pulling the fuel rod from the magazine into and through the grids of the assembly.
Loading fuel rods into the top of the fuel assembly by pulling from the bottom of the assembly works satisfactorily in the manufacturing plant; however, once the fuel assembly is in service in a reactor facility, replacement loading of a fuel rod by pulling from the bottom of the assembly becomes more difficult. This is because servicing of a fuel assembly normally takes place while the assembly is submerged in water at a work station and also due to the unavailability of loading equipment at reactor facilities which pulls fuel rods from the bottom of the fuel assembly. Therefore, when a fuel rod needs to be replaced in the reactor facility, it will usually be loaded into the assembly from the top by pushing the rod, instead of pulling it, into the fuel assembly.
However, a problem frequently arises when a fuel rod is being pushed into the fuel assembly. The lower end plug of the fuel rod, which has a relatively blunt leading surface, often gets hung up on the flow mixing vanes carried by the support grids of the fuel assembly into which the fuel rod must pass. Several competing interests make a solution to this problem difficult. On the one hand, the blunt configuration of the leading end plug surface accommodates the presence of a large open interior cavity having sufficient lateral wall structure for the gripper to enter and grab the end plug for bottom loading of the fuel rod. On the other hand, a fully tapered or "pencil point" end plug configuration would help prevent the fuel rod end plug from becoming hung up on the grid mixing vanes during top loading of the fuel rod. However, if the end plug was fully tapered, there would be insufficient lateral wall structure to define an interior cavity for the gripper to engage for effecting bottom loading of the fuel rod. Consequently, a need exists for a fuel rod end plug design which will strike a workable compromise between these competing interests and thereby facilitate loading of the fuel rod from both the bottom and top of the fuel assembly by accommodating both pulling and pushing of the fuel rod into the fuel assembly.